Batteries are used extensively in modern electronic devices, including radios, portable communications devices, such as cellular telephones, laptop computers, camcorders and the like. The term "battery" is used herein to refer generally to any device which converts chemical energy into electrical energy. Batteries are also generally used as secondary power sources in electronic devices to provide power in the event that the primary power source, for example, electric power produced by a generator, fails. The failure of primary power sources is potentially life-threatening in connection with life-sustaining electronic devices, for example, medical devices, including life-support systems. Thus, secondary power sources, including batteries, which discharge power in a reliable fashion and which have significant cycle and shelf lives are particularly important in the operation of modern electronic devices.
The term "cycle life" is used herein to refer to the number of cycles that a battery can undergo at a specified rate of discharge without significant deterioration in performance. The term "shelf life" is used herein to refer to retention of operational capabilities during storage in a charged configuration.
As known to those skilled in the art, batteries generally comprise a positive electrode (cathode) and a negative electrode (anode) which are typically separated by an intervening electrolyte. When the battery is loaded (i.e., when an external circuit from the anode to the cathode is completed), the battery is discharged and chemical energy in the battery is converted into electrical energy. The chemical reactions which are involved in the discharge of batteries include oxidation reactions which occur at the anode and reduction reactions which occur at the cathode. Accordingly, the anode typically comprises materials which are readily oxidized, for example, metals, such as lead and zinc, and the cathode typically comprises materials which are readily reduced, for example, metal oxides, such as lead oxide.
Electrically conducting polymers, for example, polyanilines, polythiophenes, and polypyrroles, offer potential advantages over conventional battery materials, including high gravimetric energy densities for maximum energy storage, high gravimetric power densities for rapid access to the stored energy, high conductivities for efficient current collection, large surface area for efficient materials utilization and high reaction rates, and low cost. In addition, the facile processability of polymers presents an improvement over traditional high-temperature metallurgical processing, and the lightweight characteristics of plastics derived from polymers generally satisfies low weight requirements associated with portable electronic devices, for example, radios, and vehicular devices, such as electrically-powered automobiles.
Interest in the application of electrically conducting polymers in batteries has been reported in the literature. See, e.g., U.S. Pat. No. 4,939,050; Japanese Patent Nos. 02239572 and 63301465; and German Patent DE 4010369. The disclosed batteries generally comprise electrically conducting polymers which have been doped with dopants, including charge-transfer agents, for example, electron donors and/or electron acceptors, such as inorganic compounds, including arsenic pentafluoride (AsFs), to increase the conductivities of the polymers. In addition, various of the batteries disclosed in these publications are solid state lithium batteries which comprise composites of doped polyaniline and carbon as the cathode. The polyaniline-carbon composite cathode may be prepared by mixing carbon, for example, carbon black, with polyaniline and pressing the mixture into a pellet.
Alternatively, the polyaniline-carbon composite cathodes may be prepared by depositing polyaniline directly on the surface of the carbon to provide polyaniline-carbon composite which is then pressed into pellets. The polyaniline-carbon composites which are pressed into pellets are referred to generally as "pelletized" polyaniline-carbon composites. The foregoing Japanese and German patents disclose further that a polymeric electrolyte, such as polyethylene, is disposed between the cathode and a compatible anode. The prior art batteries which comprise such pelletized polyaniline-carbon composites and polymeric electrolytes are depicted schematically in FIG. 1.
Batteries which comprise electrically conducting polymers, including the batteries disclosed in the foregoing Japanese and German patents and which are depicted schematically in prior art FIG. 1, suffer from various drawbacks. In this connection, prior art batteries generally possess limited cycle and shelf lives which are due to flaws inherent to the design of such batteries. These design flaws are described in detail below.
In the prior art batteries which comprise electrically conductive polymers, including those depicted in FIG. 1, electronic charge is transferred generally from the cathode to the conducting polymer in the interior of the cathode by a perculation pathway. The perculation pathway corresponds to polymer-coated carbon particles, carbon particles or a combination thereof. As the battery is discharged, the dopant gradually leaches out from the polymer, resulting in a substantial increase in the resistivity of the polymer. Eventually, the polymer behaves as an insulating material. Electronic charge must then be transferred via the carbon particles, which are generally spatially isolated from each other, and from the current collector. This results in a low or reduced utilization efficiency for cathodes which comprise pelletized polymer-carbon composites and the battery as a whole.
Attempts to overcome the reduced efficiencies of prior art cathodes and batteries which comprise pelletized polymer-carbon composites have generally been unsatisfactory. These attempts have included increasing the concentration of carbon particles which are not coated with electrically conductive polymer to provide alternative perculation pathways. However, this results in an increase of the non-active mass in the cathode, thereby decreasing the energy density of the battery.
In addition to batteries which comprise pelletized polymer-carbon composites, there is disclosed in the prior art batteries which comprise a network of stainless steel wires onto which is deposited doped polyaniline to form a composite cathode. See, e.g., U.S. Pat. No. 4,939,050. However, this arrangement is also fraught with disadvantages. For example, the surface area of the steel wire arrangement is relatively low, owing to the generally high diameters of the steel wires (35 to 120 microns). Only a limited amount of the polyaniline may therefore be deposited on the steel wire arrangement, resulting in a low or reduced utilization efficiency for cathodes and batteries which comprise such steel wire/polymer composites. Moreover, the stability of steel is limited and is prone to rusting, oxidation, and the like, resulting in cathodes and batteries which have limited shelf lives.
Thus, prior art batteries and cathodes which comprise electrically conductive polymers suffer from several drawbacks, including limited cycle and shelf lives. Moreover, the prior art batteries and cathodes generally comprise electrically conductive polymers which tend to become highly resistive, thereby reducing the utilization efficiency of the batteries and cathodes.